1. Technical Field
The present invention relates to a burnishing tool, and more particularly, to a dimple-forming burnishing tool.
2. Related Art
Generally, for slide elements (sliders) used under severe conditions of high rotational speed and heat, such as fluid dynamic bearing surfaces typified by engines and hard disk drives, there is known a technology in which, in order to improve the lubricating performance, microgrooves or dimples are formed in a sliding surface to reduce the frictional resistance of the sliding surface. Examples of the known technologies for forming dimples in a sliding surface include technologies using so-called WPC treatment (fine particle shot peening), laser beam machining, barrel polishing, etc.
On the other hand, burnishing is categorized in plastic deformation in which a work surface of a workpiece is crushed and deformed by rotating a hard roller while pressing the hard roller against the workpiece to enhance the surface hardness and surface roughness. Burnishing can greatly improve the durability, wear resistance, and reliability of the sliding surface.
As a tool of burnishing and forming dimples in an inner surface of a workpiece, known is a technique disclosed, for example, in Japanese Published Unexamined Patent Application No. 2007-301645. The dimple-forming burnishing tool disclosed in Japanese Published Unexamined Patent Application No. 2007-301645 includes a mandrel attached to a processing machine for rotation, and a retainer (a frame) externally fitted to the mandrel in a rotatable manner. The retainer rotatably holds plural rollers (rolling elements) and plural balls (pressing elements) in such a manner that the plural rollers and balls can move radially in and out of an outer surface of the retainer. Also, when the mandrel rotates, protrusions formed on an outer surface of the mandrel is brought into engagement with the rollers and the balls, thereby causing the rollers and the balls to roll while vibrating on the inner surface of the workpiece. In this manner, dimples are formed in the inner surface of the workpiece.
According to Japanese Published Unexamined Patent Application No. 2007-301645, however, a problem still exists that adjustment of a dimple shape (such as groove width, groove length, or groove depth) is difficult, due to a polygonal cross-sectional shape of the mandrel, rotation of the mandrel causes the rollers and the balls both to move radially in and out of the outer surface of the retainer at the same time. Specifically, a failure to precisely locate the tool on the inner surface of the workpiece causes a change in the distance that the rollers and the balls move radially in and out at the start of dimple formation. Therefore, stable forming processing becomes impossible, so that adjustment of the dimple shape becomes more difficult. Further, according to the tool disclosed in Japanese Published Unexamined Patent Application No. 2007-301645, a problem also still exists that dimples are redundantly formed in a tool retraction process (or at the time of tool retraction) because the tool diameter is not reduced at the time of retracting the tool after processing the inner surface of the workpiece.